A basic (very, and also naive) microservice which handles accounts and supports being able to transfer money between those accounts.
The overall requirement is simply this:
Design and implement a RESTful API (including data model and the backing implementation) for money
transfers between accounts.
Which is pretty vague (although probably by design). So for the sake of making progress at the expense of asking loads of questions I've made the following assumptions:
- Accounts are known and managed as part of this system.
- Money transfers are between internal accounts only (could do it via a client but then you have to start considering things like distributed transactions and all that joy).
- The data considerations are very basic.
- Currency is not supported (although I did add it to one of the model definitions)
- The approach for handling transfers is done using a double entry accounting approach - so we have the debit account, the credit account, and the balance is adjusted for each.
- The account balance is stored in the
Accountmodel, rather than being derived from all transactions against an account. This is more to cut down the number of models and keep it simple, although there would be performance implications in taking that approach in a production system. Accountholders can only add money to their account as part of their initial opening balance. There is no supported mechanism to add funds outside of them being transferred from someone who already has them.
Upon request a change has been made to add some safety around around the action of making a transaction between accounts. This has been done in 3 ways:
- Made the method
synchronized- while this makes the method thread safe within the application although quite obviously wouldn't help in a clustered scenario. - Added an arguably draconian transaction isolation level (
SYNCHRONIZED) - you could probably get away withREAD_COMMITTEDif you were so inclined. - Added a check to ensure that the correct number of account rows were updated and adding an explicit rollback if the expected updates are not found.
This has been built using the rather good Dropwizard which gives pretty much most of what you'd want from an HTTP based service. This bundles a Jetty instance so you don't need to worry about external containers/ servers. For persistence there is an embedded H2 instance.
The system will handle database migrations on startup so there is nothing to be run outside of just starting the service. For the migrations I used Flyway as I tend to prefer the SQL support for ease of use.
The project uses maven to manage dependencies and build everything. There is a plugin defined in the pom.xml that will build the project as part of the package command. If you want to run this from the command line (assuming java and maven are installed) you can run the following:
// From project root
mvn package
// The .jar will be under `target`. You can run it as follows
java -jar ./target/Tranzfer-1.0-SNAPSHOT.jar server ./src/main/resources/tranzfer_conf.yml
The server and location of the config files are used by Dropwizard to run and maintain the system. Nothing else should be required.
Running from within an IDE is a little less involved than running from the command line. If you're working in IntelliJ then you should click Edit configurations as part of the run menu at the top. Add a configuration for an Application, and ensure the Main class points to TranzferApplication. Once you have that you should add the following as Program arguments -> server ./src/main/resources/tranzfer_conf.yml.
No matter how you run this, the service will be running on localhost and bind to port 8080.
A high level overview of the endpoints is given in the below table. The subsequent sections will go into more detail around these:
| Endpoint | Method | Description |
|---|---|---|
| / | GET | Root of the system. Returns basic system information |
| /account | GET | Get a list of all accounts defined in the system. |
| /account | POST | Create a new account |
| /account/{account_number} | GET | Retrieve a specific account by the account number |
| /account/{account_number} | PUT | Update a particular account (holder name only) |
| /transfer | GET | Retrieve a list of all transfers currently defined |
| /transfer | POST | Initiate a money transfer between two known accounts |
| /transfer/{transfer_id} | GET | Retrieve a specific transfer record |
Hitting the root url will simply return some very basic self-promotional System Information.
curl localhost:8080
// HTTP 200
{
"authorName": "Mike Holmes",
"applicationName": "Tranzfer"
}
Hitting the /account/ root url will return a listing of all accounts currently defined in the system. If there are none you'll just get an empty array:
curl localhost:8080/account
// HTTP 200
[
{
"id": 1,
"account_number": "149b046a-b033-4132-b934-bf617e40d4ab",
"account_holder": "Mike Holmes",
"account_balance": 10000.56,
"created_at": "17-Sep-04 13:10:36"
},
...
]
You can create an account by passing an initial account object to the /account url. An account requires the account_holder field to be populated (which is the name of the person holding the account) but can also take an initial account_balance field (which currently is the only way of getting money into an account). What is returned is a far more complete account model.
curl -X POST -d "{ \"account_holder\":\"Mike Holmes\", \"account_balance\": 10000.56}" localhost:8080/account
// HTTP 201
{
"id": 1,
"account_number": "149b046a-b033-4132-b934-bf617e40d4ab",
"account_holder": "Mike Holmes",
"account_balance": 10000.56,
"created_at": "17-Sep-04 13:10:36"
}
If you need to retrieve a specific account, you can make a request to /account/{account_number} using the account_number property that would be generated when creating the account.
The root url will return a listing of all transfer records in the system:
curl lcoalhost:8080/transfer
// HTTP 200
[
{
"id": 21,
"credit_account_no": "caf85fbd-68da-4161-84ed-df9e3a00208d",
"debit_account_no": "5cac40c0-4e0c-4856-a5ab-69678ff49f4e",
"amount": 200.00,
"currency": "GBP",
"created_at": "17-Sep-11 20:41:37"
},
...
]
Rod, Jane and Freddie are friends. Each of them has an account with Tranzfer into which they have added what they have saved up. Rod has the most money:
// POST /account
{
"account_holder": "Rod",
"account_balance": 10000.00
}
Jane is a female which means that she has approximately 9.4% less money than Rod for no other reason than she is a female (which incidentally is a travesty, although not one that will be solved here) source:
// POST /account
{
"account_holder": "Jane",
"account_balance": 9060.00
}
Freddie is pretty good on the guitar but has an unfortunate penchant for faberge eggs and as such has no money. Ever.
// POST /account
{
"account_holder": "Freddie"
}
So now all 3 have an account in various degrees of financial solubility:
// GET /account
[
{
"id": 1,
"account_number": "caf85fbd-68da-4161-84ed-df9e3a00208d",
"account_holder": "Rod",
"account_balance": 10000,
"created_at": "17-Sep-04 13:21:37"
},
{
"id": 2,
"account_number": "5cac40c0-4e0c-4856-a5ab-69678ff49f4e",
"account_holder": "Freddie",
"account_balance": 0,
"created_at": "17-Sep-04 13:25:15"
},
{
"id": 3,
"account_number": "07b39e27-ab7a-4bc3-b688-7ac9a1a7e2b2",
"account_holder": "Jane",
"account_balance": 9060,
"created_at": "17-Sep-04 13:26:21"
}
]
Freddie needed a loan to purchase a particular Faberge egg that he saw on ebay and as such asked to borrow £100 from Rod. Rod agreed and transferred the money to him:
// POST /transfer
{
"debit_account_no": "caf85fbd-68da-4161-84ed-df9e3a00208d",
"credit_account_no": "5cac40c0-4e0c-4856-a5ab-69678ff49f4e",
"amount": 100.00,
"currency": "GBP"
}
We can now see that there is a single transfer defined in the system:
// GET /transfer
[
{
"amount": 100,
"currency": "GBP",
"id": 1,
"debit_account_no": "caf85fbd-68da-4161-84ed-df9e3a00208d",
"credit_account_no": "5cac40c0-4e0c-4856-a5ab-69678ff49f4e",
"created_at": "17-Sep-04 13:33:49"
}
]
Also we can see that the money has indeed been transferred between Rod and Freddie:
// GET /account
[
{
"id": 1,
"account_number": "caf85fbd-68da-4161-84ed-df9e3a00208d",
"account_holder": "Rod",
"account_balance": 9900,
"created_at": "17-Sep-04 13:21:37"
},
{
"id": 2,
"account_number": "5cac40c0-4e0c-4856-a5ab-69678ff49f4e",
"account_holder": "Freddie",
"account_balance": 100,
"created_at": "17-Sep-04 13:25:15"
}
]
Unknown to Freddie however, Rod makes his money as a loan shark charging oppressive levels of interest on relatively small amounts. As such, Rod informs Freddie that the amount owed back to him is actually now £200. Freddie tries to transfer the money to Freddie (knowing full well that he can't afford it):
// POST /transfer
{
"credit_account_no": "caf85fbd-68da-4161-84ed-df9e3a00208d",
"debit_account_no": "5cac40c0-4e0c-4856-a5ab-69678ff49f4e",
"amount": 200.00,
"currency": "GBP"
}
// HTTP 400 (Bad Request)
// Account # 5cac40c0-4e0c-4856-a5ab-69678ff49f4e does not have enough funds to make a £200.00 transfer
So Freddie had to request money from Jane to cover the oppressive interest charged by Rod:
// POST /transfer
{
"credit_account_no": "5cac40c0-4e0c-4856-a5ab-69678ff49f4e",
"debit_account_no": "07b39e27-ab7a-4bc3-b688-7ac9a1a7e2b2",
"amount": 100.00,
"currency": "GBP"
}
So now Freddie checks his account and has enough to be able to pay Rod back:
// GET /account/5cac40c0-4e0c-4856-a5ab-69678ff49f4e
{
"id": 2,
"account_number": "5cac40c0-4e0c-4856-a5ab-69678ff49f4e",
"account_holder": "Freddie",
"account_balance": 200,
"created_at": "17-Sep-04 13:25:15"
}
Freddie can send the money now to Rod:
// POST /transfer
{
"credit_account_no": "caf85fbd-68da-4161-84ed-df9e3a00208d",
"debit_account_no": "5cac40c0-4e0c-4856-a5ab-69678ff49f4e",
"amount": 200.00,
"currency": "GBP"
}
Finally we can see that Freddie is now back to having no money. Rod is the only one to have made any money out of the whole scenario. Balance has been restored.
// GET /account
[
{
"id": 3,
"account_number": "07b39e27-ab7a-4bc3-b688-7ac9a1a7e2b2",
"account_holder": "Jane",
"account_balance": 8960,
"created_at": "17-Sep-04 13:26:21"
},
{
"id": 2,
"account_number": "5cac40c0-4e0c-4856-a5ab-69678ff49f4e",
"account_holder": "Freddie",
"account_balance": 0,
"created_at": "17-Sep-04 13:25:15"
},
{
"id": 1,
"account_number": "caf85fbd-68da-4161-84ed-df9e3a00208d",
"account_holder": "Rod",
"account_balance": 10100,
"created_at": "17-Sep-04 13:21:37"
}
]